Please help!!!! What should I do?

My goldfish are not doing so great. They have been living in their 50 gallon tank for two years now and the water condition is great, the parameters are good and the water is crystal clear. However the goldfish are not so lucky. Most of my goldfish have lony white/clear poo and are flashing around, my black moore has pop eye and cloudy eye in the same eye and excess slime coat while one of my other goldfish gets columnaris every once in a while. Most of these symptoms have been recently acquired in the last month or two. They eat normally and they swim around happily with an exception to the black moore who is now becoming a little bit lethargic. I have tried anti-medicated fish food, melafix and pimafix to no avail. I currently have API Aquarium salt in my tank with the recommended amount for goldfish. PLEASE HELP!!! What medications should i use??? Sorry for the long question!